Field of the Invention
The preferred embodiments are directed to industrial transmissions and, more particularly, to a heavy-duty industrial transmission without reversing action or reverse gears and without countershafts. The present transmissions may be used in stationary applications like those within the well drilling and completion segments of the oil and gas industries, such as fracturing (frac) pump systems used for well stimulation by hydraulically fracturing subterranean formations.
Discussion of the Related Art
Hydraulically fracturing subterranean formations is a common way of increasing the porosity of and, thus, flow rate through production zones that feed boreholes of wells that remove underground resources like oil and gas. Increasing the flow rate through the production zones correspondingly increases the productivity of the wells.
Extreme hydraulic pressures are required for fracturing subterranean formations, for example, 10,000 psi or more. To achieve these pressures, frac pump systems include heavy-duty pumps that are powered by high horsepower engines, with heavy-duty transmissions delivering torque from the engines to the respective pumps.
Heavy-duty transmissions used in frac pump systems are typically already existing heavy-duty mechanical transmissions that are designed for other applications. Since these existing heavy-duty transmissions are rated to handle the high horsepower engines that drive the pumps of the frac pump systems, the existing heavy-duty transmissions are repurposed for frac pump system uses.
Commonly, these repurposed heavy-duty transmissions are from off-road or other heavy duty vehicles. Heavy-duty vehicle transmissions tend to be large and heavy with countershafts and other features which add to the overall size of the transmission housings needed to accommodate these internal components. These heavy-duty vehicle transmissions typically have a large number of speeds or ranges, such as six or more ranges, and a large number of ranges can be beneficial in frac pump systems. That is because, although only a few ranges may be used for a particular fracing application, other ranges may be used in other fracing applications, depending on the various pump sizes and well pressures that are needed for those applications. However, the ratios of the heavy-duty vehicle transmissions ranges tend to be spaced at inconsistent steps, whereby the reduction or gear ratio values of adjacent pairs of ranges tend to be different, to provide desired vehicle movement performance requirements. But inconsistent steps between ranges can make some of the ranges or groups of sequential ranges ill-suited for delivering power to high-pressure pumps of frac pump systems. Inconsistent steps can make it difficult to finely adjust pump driving speed through range changes while maintaining optimal engine RPM. Inconsistent steps may also include at least one large change in reduction ratio that the engine may not be able to power while overcoming the head pressure of the well, making the gear change too large for the engine to handle. Typical heavy-duty vehicle transmissions also include ranges that provide reversing action or reverse gears for backward travel and overdrive ranges for high speed travel. However, high-pressure pumps of frac pump systems are typically neither driven in reverse nor driven at speeds that are faster than engine speed or transmission input speed because driving the pumps at high speeds can produce heavy vibration in the equipment.
Other existing heavy-duty transmissions used in frac pump systems include heavy-duty industrial transmissions used in other stationary applications. Although some planetary-type transmissions have been implemented in frac pump systems, many industrial transmissions used in frac pump systems have countershaft configurations and a large number of gears like heavy-duty vehicle transmissions. Heavy-duty industrial transmissions for stationary applications typically have a large number of gears or ranges, though again with ratios of the ranges tending to be spaced at inconsistent steps.
In order to provide the large number of ranges, heavy-duty transmissions typically incorporate planetary gearsets or compound planetary gearsets. The planetary gearsets are controlled to direct power through different components of the planetary gearsets to provide the desired output gear ratios for ranges. Control includes actuating clutches to restrict or allow rotation of the various planetary gearset components, such as sun gears, planet gears, planet carriers, or ring gears to establish different power paths and output rotational speeds of the planetary gearsets.
Control of compound planetary gearsets or multiple planetary gearsets can require actuating multiple clutches during a single shift event, such as a multipack shift. Multipack shifts are typically synchronized and closely coordinated, and these synchronized multipack shifts can be complicated to control.
Controlling compound or multiple planetary gearsets typically cause(s) rotation of components of the planetary gearsets that are loaded and thus in the power path, but, in at least some of the ranges, also causes rotation of some components of the planetary gearsets that are not loaded.
The loaded components of planetary gearsets can experience recirculating torque in at least some ranges with higher values. Although a transmission output torque value will be a multiplication of the transmission input torque value as a function of the reduction ratio for a particular range, internal torque values can be much higher than the output torque value of the transmission. These higher internal torque values can be transmitted through at least some of the loaded planetary gearset components as recirculating torque that is higher than the output torque value(s), depending on, for example, particular gear ratios within the planetary gearsets. High recirculating torque values can affect the use-life of the components experiencing the recirculating torque and therefor require internal components that are designed to handle the high recirculating torque values.
The unloaded but rotating components of planetary gearsets are freely spinning on unloaded bearings at back-driven speeds. Some back-driven speeds can be very high. High back-driven speeds can cause roller bearings to be lightly loaded, which may result in skidding of the rollers in the bearing raceways. Skidding of the roller bearings in this way can result in high speed failures.
Another challenge with the present application is that mounting space is limited within frac pump systems, which are commonly mounted on trailers that are pulled by on-road semi tractors. In order to help repurposed heavy-duty transmissions fit in limited spaces within frac pump systems, the heavy-duty transmissions are often directly or closely coupled.
Overall, a heavy-duty transmission suitable for use in a frac pump system that allows the frac pump system to be brought smoothly online regardless of engine speed and has a relatively small size with robust construction and straightforward operation is desired.